KR20120089195A - Process for purifying chlorosilanes by distillation - Google Patents

Abstract

PURPOSE: A method for refining chlorosilanes based on distillation is provided to refine contaminated chlorosilanes, accumulate impurities, and discharge the impurities during purge streaming. CONSTITUTION: A method for refining chlorosilanes based on distillation includes the following: the boron-containing mixture of chlorosilanes(1) containing trichlorosilane, dichlorosilane(DCS), and silicon tetrachloride. The mixture is refined using a plurality of distilling columns(5, 6, 7); boron compounds of low boiling points are separated based on overhead stream containing boron-enriched DCS; and boron compounds of high boiling points are separated based on lower stream containing materials of high boiling points.

Description

PROCESS FOR PURIFYING CHLOROSILANES BY DISTILLATION [0002]

The present invention relates to a method for purifying chlorosilanes by distillation.

For example, the production of polycrystalline silicon used in the photoelectric conversion engineering or semiconductor industry begins with trichlorosilane (TCS), a raw material.

This TCS is mainly produced by three different processes:

A) Si + 3HCl → SiHCl ₃ + H ₂ + by-product

B) Si + 3 SiCl ₄ + 2H ₂ → 4SiHCl ₃ + by-product

C) SiCl ₄ + H ₂ → SiHCl ₃ + HCl + by-product

In this process, a relatively large amount of dichlorosilane (DCS) is formed in addition to other byproducts or impurities.

Thus, about 0.1 to 1.0% of DCS is known to be present in the reaction product of hydrochlorination of metal silicon according to (A).

The reaction of metal silicon with silicon tetrachloride (STC) and hydrogen (B) generally results in a much higher DCS content in the reaction product, especially when copper is used as catalyst for this process.

Also in the hydrogenation process of STC according to (C), 0.05 to 1.0% of DCS is present in the reaction product.

DCS itself is a useful product that can be used in the semiconductor industry not only for silicon deposition but also for the production of organofunctional silanes.

However, very high purity is a prerequisite here. For example, the concentration of boron should be less than 10 ppta for semiconductor applications.

Another example that may be mentioned is hydrosilylation. In the hydrosilylation process, derivatives of hydrosilanes are reacted to vinyl groups or other multiple bonds by catalytic addition reactions. Typical catalysts are complexes of precious metal platinum. Here, since boron acts as a catalyst poison, the concentration of boron should be less than 1 ppbw.

The DCS formed from the processes A-C is not suitable for this use, especially because the boron content is too high.

Since boron is a BCl ₃ with a boiling point of 8.3 ° C. and mainly has a boiling point close to the boiling point (12.5 ° C.) of the existing DCS, boron is virtually completely concentrated in the DCS product stream during subsequent distillation processes.

Despite the difference in boiling point of about 30K, the separation of BCl ₃ from TCS by distillation is incomplete, especially when a boron content of <0.1 ppm in TCS has to be achieved.

In the prior art, BCl ₃ produced in the hydrochlorination reaction of metal silicon is withdrawn from the system with a predetermined amount of trichlorosilane. This is described, for example, in "Handbook of Semiconductor Silicon Technology", William C. O'Mara, Robert B. Herring and Lee P. Hunt, Noyes Publications, USA 1990, page 4, fig. 2 is described.

Due to the very similar boiling point, DCS is also released from the system along with BCl ₃ , which lowers the overall plant economy.

Four different approaches are known which are essential for removing boron impurities from TCS.

Thus, purely distillation processes and processes with hydrolysis or adsorption steps have been used.

In patent document DE 10 2007 014 107 A1, boron-depleted chlorosilanes are obtained from a boron-containing chlorosilane mixture by removing the boron-enriched distillation stream by distillation, and the boron-enriched side stream is A method is described in which one or more distillation columns are branched from one or more distillation columns arranged and disposed of or sent for further use. By withdrawing the product from the various column arrangements and overhead and side offtakes on each column, the boron content in pure DCS in the substream can be reduced to about 50 ppm. However, the boron concentration is increased even more in another substream containing DCS and TCS. Another disadvantage is that there is not much amount of TCS removed as waste.

Patent document DE 10 2008 002 537 A1 discloses a method for reducing the boron content in a composition I comprising at least one silicon halide, wherein in a first step, the composition I is in contact with up to 600 mg of water per kg of composition I, Composition I, in contact with moisture in the first step, is optionally transferred completely or partially, one or more times to the separation and removal of the hydrolyzed boron- and / or silicon-containing compound to form pre-purified composition II, which Composition II is either completely or partially returned to the first stage or fed to the second stage of the process, and the hydrolyzed boron- and / or silicon-containing compound is separated off by distillation in the second stage, whereby the distillate A method for forming prerefined composition II with reduced boron content as distillate is disclosed.

The boron content in chlorosilanes can be reduced, for example, by the addition of water in a suitable form. The reaction of boron halide with water forms a high boiling hydrolyzate, which can be more easily separated from chlorosilanes by distillation. However, these processes require additional purge streams to separate boron and the formed chlorosilane hydrolyzate (eg,> 5% purge streams based on the starting material). Corrosion due to the deposition of silica and the HCl formed of the plant components is also a problem. Corrosion subsequently leads to the release of dopants such as P and As from the steel of the plant.

Patent document EP 2 036 858 A2 claims a method of contacting boron- and phosphorus-containing chlorosilanes with benzaldehyde and oxygen, which are complexing agents. As a result of oxidation and complex formation, the boron compounds present in the chlorosilanes can be easily separated. However, as described in Example 6 of this patent application, a residue of about 10% is obtained which must be discharged with the boron complex. Due to the relatively slow response (30 minutes), this method is not suitable for continuous operation. In addition, the stirring vessel increases the cost associated with the device and introduces organic contaminants into the target product.

Patent document DE 10 2008 054 537 discloses a method for treating a composition containing at least one silicone compound and at least one foreign metal and / or a compound containing a heterogeneous metal, the composition comprising: Contacting at least one adsorbent and / or at least one first filter and, optionally, in further steps, contacting at least one filter to provide a composition having reduced content of the heterogeneous metal and / or the compound containing the heterogeneous metal. The method is described.

Here, the boron content in chlorosilane is reduced by contact with anhydrous adsorbent. However, very large amounts of adsorbent (120 g / TCS 250 ml) are required to achieve the desired purification effect. This makes the process uneconomical, in particular because the continuous process is virtually unsuitable, which is economically disadvantageous in producing chlorosilanes of semiconductor quality. The use of adsorbents also requires additional equipment (e.g., filtration equipment) and poses the risk of introducing other impurities into the semiconductor-pure product.

In view of the above problems, it is an object of the present invention to purify contaminated chlorosilanes at reduced expense, to accumulate impurities and to discharge them into an ideally small purge stream. On today's economic scale, the yield of TCS should be significantly higher than 95%.

The purely distillation process has been found to be advantageous because it does not require additional equipment and can be run continuously in a convenient manner. Loss of chlorosilanes can be best minimized in this process.

The advantage of the distillation process is that the risk of introducing additional impurities is very low.

It is an object of the present invention to provide a boron-containing mixture of chlorosilanes containing TCS, DCS and STC as a process for purifying chlorosilanes by distillation, and distilling the mixture of chlorosilanes in a plurality of distillation columns. Purifying by, and branching the low-boiling boron compound from the distillation column using an overhead stream containing boron-rich DCS, and the boron-rich bottom stream containing high boiler. It is achieved by the purification method of chlorosilane which branches a high boiling point boron compound using.

The mixture of chlorosilanes provided is preferably prepared by reaction of metal silicon with HCl at 350-400 ° C. in a fluidized bed reactor.

The mixture of chlorosilanes provided is preferably fed to a separation column, where the column parameters are less than 10 ppm STC in the first fraction from this separation column and also less than 10 ppm TCS in the second fraction from this separation column. Is selected to be present.

Relevant column parameters are, in particular, the pressure and temperature at the bottom and the number of theoretical plates.

The second fraction from the separation column is preferably fed to a second column and separated by distillation into an overhead stream containing STC and a boron-rich bottom stream containing high boiling point material.

The first fraction from the separation column is preferably fed to a third column and is boron-rich overhead overhead stream containing TCS with a low boiler such as DCS and a bottoms stream containing TCS by distillation. Separated by.

The overhead stream containing the TCS together with the low boiling point material such as DCS from the third column is fed to the fourth column fed with inert gas and the overhead stream containing boron-rich DCS is withdrawn from the fourth column. The bottoms stream from the fourth column is recycled to the separation column and the second stream containing offgas from the fourth column is discarded.

The fourth column is preferably operated at a pressure higher than atmospheric pressure.

The overhead stream containing the low boiling point material such as DCS from the third column and the TCS is preferably liquefied before being fed to the fourth column.

According to the present invention, there is provided a process for purifying chlorosilanes in which contaminated chlorosilanes are purified at reduced expense, and impurities are accumulated and discharged in an ideally small purge stream.

1 is a flowchart of a process for work-up of a chlorosilane mixture by distillation.
2 is a schematic diagram illustrating a condensation process of an overhead product from a distillation column.

1 shows the principle of workup by distillation of a chlorosilane mixture obtained as the product of the hydrochlorination reaction of metal silicon. The essential purpose is the separation of boron and phosphorus impurities from the target product TCS.

FIG. 2 shows the condensation process of the overhead product from the distillation column 6 shown in FIG. 1.

The overhead product is in turn cooled by a water cooler 6w, a brine cooler 6s and a low temperature cooler (Frigen) 6t. In each case the condensate formed is reused. Condensate from the water cooler is recycled to the column. Condensate from the brine and low temperature cooler is fed to column 7 as product stream 6a. Offgas is discarded.

The present invention is based on a comprehensive analytical study of the distribution of boron impurities in the various chlorosilane fractions of an integrated plant for producing polycrystalline silicon.

An important step of the present invention is the preparation of chlorosilanes, preferably TCS by hydrochlorination of metal silicon, purification of chlorosilanes by distillation, and of highly contaminated DCS and STC fractions by boron from mixtures of these chlorosilanes. Removal step.

Effective removal of boron from the TCS resulting from the hydrochlorination process of the metal silicon can be achieved according to the invention by distillation using an array of various columns as described below.

The purpose is to concentrate the high boiling boron compound in the STC substream and the low boiling boron compound in the DCS substream.

This makes it possible to avoid TCS in the waste at the same time as producing TCS containing less than 20 ppb of boron.

86% TCS, 13.5% STC, 0.3% DCS, 3.2 ppm boron and trace additional impurities (methylchlorosilane, obtained by reaction of commercial metal silicon with HCl at a temperature of 350-400 ° C. in a fluidized bed reactor A mixture of chlorosilanes 1 (FIG. 1) containing hydrocarbons, high boiling materials such as siloxanes and disilanes) is fed to the separation column 2.

Here, the column parameters are selected such that the overhead product 3 contains less than 10 ppm STC and the bottom product 4 contains less than 10 ppm TCS.

The bottom product 4 is fed to another column 5, where the STC fraction 5a and the high boiling fraction 5b (e.g. siloxane, disilane, methyltrichlorosilane, and possibly metal chlorides) Separated by.

Since the high boiling point compound 5b occupies only about 1% of the total amount, it is separated continuously or batchwise from the bottom of the column.

The overhead product 3 from the column 2 is separated in the next column 6 into a fraction 6b containing clean TCS and a fraction 6a containing TCS together with a low boiling point material.

An additional stream of contaminated DCS containing TCS contains an overhead product (3) from column (2) before column (6) if the stream contains an amount that is negligible to neglect the component having a boiling point lower than TCS. Can be introduced within.

The fraction 6b can be used for further workup.

The fraction 6b contains not only DCS but also a significant amount of low boiling impurities such as TCS and BCl ₃ .

This fraction is fed to an additional column 7 and in a particularly preferred variant, an inert gas can be further supplied. The column 7 is designed to be operated at a pressure higher than atmospheric pressure.

Bottom product 7b from column 7 is recycled for use in column 2.

The offgas 7c from the column 7, which contains a significant amount of boron, can be sent to a further disposal step through a scrubber.

The overhead product 7a from column 7 contains a high proportion of boron contaminants as well as DCS.

Thus, this stream serves to effectively drain boron contaminants from the system.

As shown in the examples below, a dramatic reduction in the boron content of the TCS stream is surprisingly obtained when the overhead product is liquefied from column 6 by multistage cooling and the condensate of the reactive cooling stage is properly transferred ( 2).

It has been found to be particularly useful to cool the overhead product from column 6 to a temperature of about 10-30 ° C., preferably 15-25 ° C., first by means of a water cooler 6w.

Condensate 6wk from this cooler is recycled to the column.

Uncondensed material 6wnk is fed to the brine cooler 6s, where the product stream is cooled to about -7 ° C.

Condensate 6sk from this brine cooler forms the first component of stream 6a.

The non-condensed material 6snk in the brine cooler is fed to the low temperature cooling stage 6t, where it condenses to form condensate 6tk.

This forms the second component for stream 6a. The low temperature cooling stage cools the product stream to about -60 ° C. Here again the uncondensed material is discarded as offgas. The whole product stream 6a is fed to column 7.

Substream 6b from the described process is the target product of the integrated chlorosilane plant, ie purified TCS, for the production of polysilicon.

The TCS prepared in this way can be used directly for the deposition of solar cell quality polysilicon, in a mixture with other chlorosilane streams, or purified to semiconductor quality by other distillation steps.

Example

86% TCS, 13.5% STC, 0.3% DCS, 3.2 ppm boron and traces of additional impurities (methylchlorosilane, hydrocarbons, siloxane and The chlorosilane mixture (1) having a composition of a high boiling point material such as disilane) was worked up by distillation at a temperature of 350 to 400 ° C. in a fluidized bed reactor.

Overhead stream 3 from column 2 was found to contain 3.4 ppm boron (mostly volatile BCl ₃ ) and bottom stream 4 contained 1.1 ppm high boiling boron compound.

The STC stream 4 was distilled off in column 5 and the high boiling point material was separated through the bottoms stream; Removal of the boron compound is incomplete because the overhead stream 5a still contains 1 ppm of boron.

Comparative example  One

Chlorosilane stream (3) was distilled off in subsequent column (6). This distillation process was carried out according to the prior art, ie simply by discharging boron impurities with a predetermined amount of chlorosilanes.

Here, the column parameters were chosen such that pure DCS was distilled at the top while TCS was withdrawn from the bottom of the column.

TCS distilled in this manner still contained 280 ppbw of boron compound.

It has been found that the boron compound cannot be completely separated with DCS from trichlorosilane.

Comparative example  2

Chlorosilane stream (3) was distilled off in subsequent column (6). This distillation process was also carried out according to the prior art, ie simply by discharging boron impurities with a predetermined amount of chlorosilane.

Column parameters were chosen such that a mixture of 10% DCS and 90% TCS was withdrawn at the top.

TCS withdrawn from the bottom still contained 14 ppbw of boron.

However, 27 kg / h of TCS was lost based on 860 kg of TCS used.

Example  3

The low boiling point material containing TCS fraction 3 was distilled off in column 6 and the amount withdrawn from the top was chosen such that the concentration of DCS in the overhead product 6a was 10%.

A boron concentration of 88 ppm was confirmed at 6a, and the bottom product from this column 6b contained 17 ppm of boron.

According to the described process, at least 99% of the low boiling boron compounds can be separated through the overhead product.

The DCS-containing fraction 6b was distilled off in column 7 under a meter pressure of 0.1-2.5 bar.

Pure TCS containing less than 10 ppm DCS and 2.6 ppm boron was obtained in the bottom product (7b). This product was recycled to column (2).

99.4% DCS, 0.6% monochlorosilane and 770 ppm boron were found in the overhead product 7a.

After all the impurities were separated off, about 83% of pure TCS containing less than 20 ppb of boron could be prepared from the chlorosilane mixture (1).

Recycling the TCS fraction (7b) increased the yield to 86%.

In addition, the STC fraction 5a is obtained in an amount of about 13% based on the amount of the starting mixture, has a boron content of 1 ppm, and the DCS fraction 7a is in an amount of about 0.3% based on the amount of the starting mixture. Obtained, containing about 770 ppm of boron.

Example  4

The low boiling point material containing TCS fraction (3) was distilled as described in Example 3.

However, _{20 m} 3 / h of nitrogen 6c having a residual moisture content of less than H ₂ O 1 ppmv was further introduced into the DCS-containing feed stream 6a fed to the column 7.

Ar or H ₂ could also be used as the inert gas.

The point of introduction of the inert gas can be either in the feed stream or in the column itself. For the purposes of the examples, an inert gas was introduced into the feed stream.

In addition, low temperature condensation of the offgas was omitted.

By adding an inert gas, the amount of offgas from the column was increased.

Only 400 ppm boron was found in the overhead product 7a.

In theory, it was expected that at least twice that amount of boron, ie at least 50% of the BCl ₃ supplied to the column would accumulate in the offgas stream 7c.

The offgas stream containing mostly nitrogen and trace amounts of BCL ₃ , MCS and DCS was transferred to a scrubber and discarded.

The TCS 6b prepared in this example contained only 12 ppbw of boron.

The results are summarized in Table 1.

 TCS yield B content of TCS Comparative Example 1 96.5% 14 ppbw Comparative Example 2 100% 280ppbw Example 3 100% 17ppbw Example 4 100% 12ppbw

Example  5

The low boiling point material containing TCS fraction (3) was distilled as described in Example 3.

Here, the brine condensate from column 6 was discharged to column 7 (see FIG. 2).

Surprisingly, this method reduces the concentration of impurities more dramatically without reducing the TCS yield.

It is clear that this method has a positive effect not only on the boron concentration but also on the phosphorus concentration.

The results are summarized in Table 2.

In Example 5, there is a significant improvement over Example 3 in terms of phosphorus contamination as well as boron contamination at a constant TCS yield.

TCS yield boron sign Example 3 100% 17ppbw 16.2ppba Example 5 100% <5 ppbw 3.1 ppba

1 chlorosilane; Two separation columns; 3 overhead product; 5, 6, 7 columns; 6a product stream; 6w can cooler; 6s brine cooler; 6t low temperature cooler; 6s brine cooler; 6sk, 6wk condensate; 6snk, 6wnk uncondensed material; 6t low temperature cooling stage; 6tk condensate; 7b bottom product; 7c offgas

Claims (9)

As a method for purifying chlorosilanes by distillation,
Providing a boron-containing mixture of chlorosilanes containing TCS, DCS and STC, and purifying the mixture of chlorosilanes by distillation in a plurality of distillation columns,
Low boiling boron compounds are branched from the distillation column using an overhead stream containing boron-enriched DCS and high boiling using a boron-enriched bottom stream containing high boilers. Branching point boron compound,
Method for Purifying Chlorosilane. The method of claim 1,
The mixture of chlorosilanes provided is prepared by the reaction of metal silicon with HCl at a temperature of 350-400 ° C. in a fluidized bed reactor. The method of claim 1,
The mixture of chlorosilanes provided is fed to a separation column (2), the column parameters being less than 10 ppm STC in the first fraction (3) from the separation column (2) and also the separation column (2) A method of purifying chlorosilanes, wherein less than 10 ppm of TCS is selected in the second fraction (4) from. The method of claim 3,
The second fraction (4) from the separation column (2) is fed to a second column (5) and is boron-rich with high boiling point material and overhead stream (5a) containing STC by distillation A process for purifying chlorosilanes, separated by bottoms stream (5b). The method of claim 3,
The first fraction (3) from the separation column (2) is fed to a third column (6) and contains TCS with a low boiling point material such as DCS and a bottoms stream (6b) containing TCS by distillation. A process for purifying chlorosilanes, which is separated into a boron-enriched overhead stream (6a). The method of claim 5,
The overhead stream 6a from the third column 6 is fed to a fourth column 7 to which an inert gas is supplied, and containing boron-rich DCS from the fourth column 7. Stream 7a is withdrawn and bottoms stream 7b from the fourth column 7 is recycled to the separation column 2 and a second stream containing offgas from the fourth column 7 ( 7c) is discarded. The method of claim 6,
The fourth column (7) is operated under pressure higher than atmospheric pressure. The method according to claim 6 or 7,
The overhead stream (6a) from the third column (6) is liquefied before being fed to the fourth column (7). The method of claim 8,
The overhead stream 6a is cooled to a temperature of about 10-30 ° C. by the water cooler 6w, the formed condensate 6wk is recycled to the third column 6, and the uncondensed material 6wnk. Silver is fed to a brine cooler 6s that cools the product stream to about −7 ° C., and the non-condensed material 6snk in the brine cooler 6s is fed to a cold cooling stage 6t, where Condensate (6tk) and condensate (6sk) from the brine cooler are fed to the fourth column (7).

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